Combustor liner and heat shield assembly

ABSTRACT

Impingement hole patterns are provided in a combustor dome panel to provide efficient cooling of a combustor dome heat shield. The impingement holes are substantially aligned or located above partial height protuberances provided on the back face of the heat shield.

TECHNICAL FIELD

The invention relates generally to gas turbine engine combustors and,more particularly, to combustor heat shield cooling.

BACKGROUND OF THE ART

Heat shields, which protect the dome panel of combustor shells, areexposed to hot gases in the primary combustion zone. The amount ofcoolant available for cooling the heat shields must be minimized toimprove the combustion efficiency and to reduce the smoke, unburnedhydrocarbon and CO/NOx emission. Example heat shields are shown in U.S.Pat. Nos. 4,934,145 and 5,419,115.

There is a continuing need for improved heat shields and coolingschemes.

SUMMARY

In one aspect, there is provided a combustor and dome heat shieldassembly comprising a plurality of heat shield sectors mounted adjacentand spaced apart a distance from an annular dome of the combustor, eachheat shield sector having a heat shield body defined between opposedlateral edges and radially inner and outer edges, at least one fuelnozzle opening defined in the heat shield body, the heat shield bodyhaving a back face facing the combustor dome, the back face having afirst plurality of protuberances extending from the back face to a firstheight, the first height being substantially equal to said distance, theback face having a second plurality of protuberances extending from theback face to a second height, the second height being less than thefirst height, the combustor having a plurality of impingement coolingholes defined through the dome, the impingement cooling holessubstantially aligned with the second plurality of protuberances.

In a second aspect, there is provided a combustor dome comprising atleast one heat shield mounted to an annular dome panel, the at least oneheat shield and the dome panel defining an air space therebetween and atleast one fuel nozzle opening for receiving a fuel nozzle, a pluralityof impingement holes defined in said dome panel for causing animpingement fluid to impinge upon a back face of the at least one heatshield, the back face having a heat exchange promoting protuberancepattern thereon, said pattern being divided in regions by ribsprojecting outwardly from said back face, said ribs including a firstrib extending around said fuel nozzle opening and circumscribing acentral annular area and a second rib extending from opposed sides ofthe first rib towards lateral edges of the at least one heat shield, theheat exchange promoting protuberance pattern including first and secondset of protuberances, the protuberances of the second set being shorterthan the protuberances of the first set and located at least partlyoutwardly of the central annular area adjacent to the first rib andadjacent to the second rib on either sides thereof, and wherein theimpingement holes are substantially aligned with the second set ofprotuberances.

Further details of these and other aspects will be apparent from thedetailed description and figures included below.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures, in which:

FIG. 1 is a schematic cross-sectional view of a turbofan engine having areverse flow annular combustor and dome panel heat shields;

FIG. 2 is an enlarged view of a combustor shell of the engine combustorshown in FIG. 1;

FIG. 2 b is an enlarged view of FIG. 2;

FIG. 3 is an outside end view of the dome panel of the combustor shell,illustrating an impingement hole pattern;

FIG. 4 is a perspective view of a back face of a dome heat shield of thecombustor;

FIG. 5 is a plan view of a back face of the heat shield shown in FIG. 4;

FIG. 5 b is a view similar to FIG. 5 a, showing impingement areas andpin fin density regions;

FIG. 6 is a sectional view of the portion of FIG. 5 indicated 6-6; and

FIG. 7 is a sectional view of the indicated portion of FIG. 5 indicated7-7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, amultistage compressor 14 for pressurizing the air, a combustor 16 inwhich the compressed air is mixed with fuel and ignited for generatingan annular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases.

The combustor 16 is housed in a plenum 17 supplied with compressed airfrom compressor 14. As shown in FIG. 2, the combustor 16 comprises areverse flow annular combustor shell 20 composed of a radially innerliner 20 a and a radially outer liner 20 b, defining a combustionchamber 22. The combustor 16 has a bulkhead or inlet dome portion orpanel 24 and an exit potion 26 for communicating combustion gases withthe turbine section 18. As shown in FIG. 1, a plurality of fuel nozzles28 are mounted to extend through the inlet dome end portion 24 of thecombustor 20 to deliver a fuel-air mixture to the chamber 22.

A plurality of effusion holes (not shown) are preferably defined in theinner and outer liners 20 a and 20 b for cooling purposes, and dilutionholes (not shown) are also preferably provided for combustion purposes.Inner and outer liners 20 a and 20 b may have any suitableconfiguration, and thus are shown in dotted line only in FIG. 2. Theinner and outer liners 20 a and 20 b are preferably made out of sheetmetal, though any suitable material(s) and manufacturing method(s) maybe used. A thermal barrier coating (not shown) is preferably applied tothe inner or combustion facing surfaces 32, 34 of the liners 20 a and 20b to protect them against the high temperatures prevailing in thecombustion chamber 22.

As shown in FIG. 2, the inner and outer liners 20 a and 20 brespectively include flanges 38 and 36 which overlap each other so as toform the dome panel 24 of the combustor shell 20 (Alternatively, anyother suitable dome panel could be employed). The flanges 36 and 38 aredirectly fixedly secured together by a plurality of circumferentiallydistributed dome heat shields 40 mounted inside the combustion chamber22 to protect the end wall of the dome 24 from the high temperatures inthe combustion chamber 22. The dome heat shields 40 are typicallycastings made out of high temperature materials. Each dome heat shield40 has a plurality of threaded studs 42 (four according to the exampleshown in FIG. 4) extending from a back face of the heat shield andthrough holes 44 (FIG. 3) defined in flanges 36 and 38. Self-lockingnuts 46 are threadably engaged on the studs 42 from outside of thecombustion chamber 22 for holding flanges 36 and 38 (and thus inner andouter liners 20 a and 20 b), and the dome heat shields 40 tightlytogether.

As shown in FIGS. 2 and 3, fuel nozzles openings 48 are defined throughthe dome panel 24 for allowing mounting of the fuel nozzles 28 to thecombustor 16. A central hole 52 is defined in each of the heat shields40 and is aligned with a corresponding fuel nozzle opening 48 foraccommodating an associated fuel nozzle therein. As illustrated in FIG.2, a floating collar 54 is mounted in the nozzle opening 48 to providesealing between the combustor shell 20 and the fuel nozzles 28 whileallowing relative movement therebetween. The floating collar 54 has ananti-rotation tab (not shown) which fits within an anti-rotation slot 55on heat shield 40 (see FIG. 4.) The fuel nozzles 28 are slidablyreceived in the floating collars 54. The floating collars 54 are axiallytrapped between the heat shields 40 and the end wall (i.e. flange 36) ofthe combustor dome 24. The fuel nozzle openings 48 are slightlyoversized relative to the floating collars 54, thereby allowing limitedradial movement of the collars 54 with the fuel nozzles 28 relative tothe combustor shell 20.

As shown in FIG. 2 b, the heat shields 40 are spaced from the dome panel24 by a distance of about 0.1 inch so as to define a heat shield backface cooling air space 60. Relatively cool air from plenum 17 isadmitted in the back cooling air space 60 via impingement holes 62defined in the dome panel of the combustor shell 20 (see FIG. 3). Theimpingement hole patterns are arranged in the dome panel of thecombustor shell 20 to optimize the heat shield cooling, in co-operationwith pin fins located on the heat shield, as will be described furtherbelow. As shown in FIG. 3, the impingement holes include a first set ofholes 62 a arranged on two circular paths concentrically arranged withfuel nozzle opening 48. Preferably the inner circle of set 62 acomprises holes equally spaced on a first pitch about the nozzle opening48, while the outer circle of set 62 a comprises only 10 holes (6 outerand 4 inner) but on a pitch similar to the first pitch. Placement of theouter circle of set 62 a will be discussed further below. Holes 62 b arealso provided in two rows extending laterally from each side of nozzlehole 48, both rows concentric with the central axis of the annularcombustor 20. Preferably the holes 62 of inner row have an angularposition which is staggered relative to the holes of the outer row ofthe set of 62 b. Preferably 8 holes are provided in the outer row, and 7holes 62 on the inner row, in each set 62 b on either side of nozzlehole 48. Though this is the preferred embodiment, other hole placementsand numbers maybe used. Placement of the holes 62 of set 62 b will alsobe discussed further below. Holes 62 are preferably straight-throughholes generally perpendicular to the dome panel face, thus having anaxis generally parallel to the combustor (or engine) axis. By placingholes sets 62 a and 62 b in circular arrays allows the holes to be laserdrilled using drilling-on-the-fly (DOF) techniques, which speedsmanufacturing time. As will be discussed further below, impingementholes 62 are positioned and arranged directly (i.e. generallyperpendicularly) above reduced-height pin fins 64 a on the back face ofthe heat shield to improve cooling by minimising the resistance to theair flow, which facilitates combustor cooling where a low pressure dropor gradient is available to energize combustor cooling. This allows foran optimized cooling to be achieved on the heat shield while stillproviding enough momentum to the air exiting from behind the heat shieldto form a uniform film around the circumference of the surfaces 32 and34 of the inner and outer liners.

FIGS. 4 and 5 show an individual dome heat shield 40. Each heat shield40 is provided in the form of a circular sector having a radially innerlip 41, having a plurality of ribs 72 discussed further below, aradially outer lip 43 and lateral edges 45, 47 extending between theinner and outer lips 41 and 43. Heat exchange promoting protuberances,such as pin fins 64, pedestals or other raised cooling structures, areprovided preferably in rows, and preferably in staggered position fromrow to row, on the back face of the heat shields 40 for augmenting theheat transfer between the back face and the cooling air. The pin findensity and location are defined based on the heat shield hot spots andto minimize the pressure drop, as will be discussed further below. Aswill be discussed further below, the pin fins 64 have different heights,depending on their location on the back face of the heat shield 40. Thepin fin 64 height is preferably substantially the same as the distancebetween the heat shield back face and the inner surface of the domepanel (e.g. in this example, about 0.095″ to 0.1″ from the back face).The pin fin-to-pin fin spacing is based on required cooling, and in thepresent embodiment ranges from 0.05 inch to 0.1 inch. Each pin fin 64preferably has a frustoconical shape.

As shown in FIGS. 4 and 5, ribs or ridges 66 are provided extending fromthe back face of the heat shields 40 to strengthen the heat shield anddirect the flow of cooling air as desired, as will be discussed furtherbelow. The ridges 66 preferably extend from the heat shield back faceall the way into substantially sealing contact with the inner surface ofthe dome panel (e.g. in this example, about 0.095″ to 0.1″ from the backface), and thus more or less sealing engage the dome panel and therebydirect the cooling air from impingement hole sets 62 a and 62 b to thevarious regions of the heat shield, as will be described further below.The ridges 66 include a central circular ridge 66 a concentricallydisposed with the fuel nozzle opening 52, a pair of generallydiametrically opposed primary ridges 66 b extending laterally from thecentral circular ridge 66 a, a pair of generally radially disposedridges 66 c extending radially outwardly from the central circular ridge66 a, and a pair of generally radially disposed ridges 66 d extendingradially inwardly from the central circular ridge 66 a.

As shown in FIG. 2, the central circular ridge 66 a preferably extendsaround fuel nozzle opening 52 in the heat shield in sealing contact withthe inner surface of the dome panel. Referring to FIG. 5 b, the areas ofimpingement by air passing through holes 62 of sets 62 a and 62 b areindicated by corresponding ellipses overlaid on the heat shield 40. Ascan be seen from FIG. 5 b, the rows of impingement holes 62 a align withone on either side of the central circular ridge 66 a. Outer circle ofholes 62 of set 62 a generally align with the short pin fins 64 aadjacent the central circular ridge 66 a. Referring again to FIGS. 4 and5, the cooling air from holes 62 of the inner circle of the set 62 aimpinges upon the portion of the back surface of the beat shield 40bounded by circular ridge 66 a and is then mostly directed into coolingholes 67 extending through the beat shield 40 for exhausting through theface of the heat shield. A portion of the cooling air, however, isdirected through (preferably) four grooves 63 defined at the radiallyouter side of the heat shield through the circular ridge 66 a. Whenmated against the combustor dome panel, the grooves 63 provide coolingholes or slots for allowing a portion of the cooling air to bedischarged through the grooves 63 towards the radially outer lip 43 ofthe heat shield 40, as shown in FIG. 5, and thereby cool an adjacentarea 81 where no pin fins 64 are provided, due to space limitations onthe heat shield back face for a given dome panel and fuel nozzlegeometry. Grooves 63 also permit a proper radial airflow to exit theback of the heat shield and into the combustion chamber 22 (e.g seeupper arrows a1 in FIGS. 2, 2 a). The cooling air from holes 62 of theouter circle of the set 62 a impinges upon the portion 80 a (FIG. 5) ofthe back surface of the heat shield 40, and thus tends to be directedgenerally radially outwardly or radially inwardly, as the case may be,by the ridge 66 a in co-operation with ridges 66 c or 66 d, as the casemay be, due to the substantially sealing contact provided by the ridges66 with the combustor dome panel. Also as shown in FIG. 4 (only), acircular array of short pin fins 64 a may optionally be provided withincircular ridge 66 a.

Referring back to FIG. 4, ridges 66 b extend laterally from the centralcircular ridge 66 a such as to divide the back surface area of the heatshield 40 into a radially outer half 68 and a radially inner half 70(the term “half” is used approximately). Ridges 66 b preferably extendparallely to impingement holes 62 of set 62 b, and disposed to beintermediate the inner and outer circle of holes 62 of set 62 b, as canbe seen with reference to FIG. 5 b. Inner and outer circle of holes 62of set 62 b generally align with a first row of the short pin fins 64 aimmediately adjacent ridges 66 b. Thus, the two rows (i.e. inner andouter circles) of impingement holes 62 of the set 62 b in the domepanels are located one on either side of the ridges 66 b. The air fromimpingement holes 62 b impinges upon the portions 80 b of the back faceof the shield adjacent the ridges 66 b, and thus tends to be directedgenerally radially outwardly or radially inwardly of the ridges 66 b, asthe case may be, due to the substantially sealing contact provided bythe ridges 66 with the combustor dome panel.

As mentioned, the ridges 66 c extend generally radially outwardly fromopposed sides of the central circular ridge 66 a towards, but stoppingpreferably short of, the radially outer lip 43. Likewise, the ridges 66d extend generally radially inwardly from opposed sides of the centralcircular ridge 66 a towards, but stopping preferably short of, theradially inner lip 41. The ridges 66 c are thus preferably generallyaligned with the ridges 66 d, and bound regions 80 a, for radiallydirecting cooling air in that region.

As mentioned, and shown in FIGS. 4 and 5, the heat shield 40 providedwith arrays of “full height” pin fins 64 (i.e. extending substantially,but preferably not quite, the entire distance between the heat shieldback face and the combustor dome panel, or about 0.090″ to 0.1″ in thisexample, and more preferably to 0.090″ to 0.095″), however, in regions80 a and 80 b (see FIG. 5), adjacent to ridges 66, partial height pinfins 64 a are provided. Preferably, partial height pin fins 64 a areabout one half of the height of full-height pin fins 64, but otherwisehave the same shape and configuration (i.e. preferably partial heightpin fins 64 a appear as a “sawed off” version of pin fins 64). The pinfin height is reduced to improve the impingement cooling effectivenesswhile maximizing surface area for heat transfer. The ratio (R_(dh)) ofthe diameter of cooling hole to the height from the impingement surfaceshould preferably greater than one and less than five (i.e. 1<R_(dh)<5)for maximum impingement cooling effectiveness. Depending on pin findensity in the impingement zone, the height from the impingement surfacemay be considered to be the distance from the impingement holes toeither the tops of the pin fins, the heat shield back face surface, or asuitable averaging of the two. Typically, the first (pin fin tops) willbe used. Thus, this desired requirement would not be met with afull-height pin fin 64, but in the current embodiment, the pin fins 64 aand holes 62 can be respectively sized such that an optimum impingementheight is achieved and an increased cooling surface area can still beprovided in the impingement regions 80 a, 80 b of impingement holes 62.In the present example, the tops of reduced-height pin fins 64 a are inthe range of 0.045-0.055 below the impingement holes, and theimpingement holes have a diameter in the range of 0.025-0.035, thusproviding an R_(dh) in the range of about 1.3 to 2.2 or, generallyspeaking, 1<R_(dh)<3.

An area (unindicated) of pin fins 64 adjacent anti-rotation slot 55 mayrequire height reduction to some extent, to avoid interference with theanti-rotation tab of floating collar 54.

The skilled reader will appreciate that, in general, a higher pin findensity will increase surface area, and thus generally increase heattransfer. However, in situations were insufficient flow is available toovercome the additional flow resistance provided by increased pin findensity, improvements are needed to augment heat transfer. Referring toFIG. 5 b, at hot spots regions of the heat shield, such as theperipheral regions 68 a and 70 a, the pin fin 64 density is preferablyreduced, relative to central regions 68 b and 70 b, to increase the heattransfer coefficients by increasing the coolant flow in these peripheralareas, by reducing flow resistance by reducing pin fin density toincrease the flow. Preferably, pin fin densities in regions 68 a and 70a are between 0.4 and 0.7 of the densities in regions 68 b and 70 b,respectively, however the exact densities will be determined based oncooling air flow and heat transfer requirements. For example, pin findensities for regions 68 a, 68 b, 70 a, and 70 b may be 144, 250, 170,289 respectively. Due to lower overall pressure drop experienced in thehot spot regions 68 a and 70 a due to lower pin fin densities, the heattransfer is optimised by directing more coolant flow through theseregions than would be possible if higher densities were used. It isnoted that in this example, studs 42 correspond to regions 68 a and 70a, although this is not necessary.

Lateral ridges or ribs 69 and 71 are provided at lateral edges 45, 47 ofeach heat shield 40 provide a means for redirecting the flow of coolingair behind the heat shield away from the interface of mating sides ofadjacent heat shields 40, and thus impede leakage between adjacent heatshields. The cooling air directed through impingement holes 62 or set 62b is, thus, preferably eventually fully exhausted at the inner and outerlips 41 and 43 of each heat shield 40. As shown in FIG. 4, straightenerribs 72 can be provided along the inner and outer lips 41 and 43 tostraighten the cooling air flow before being discharged in thecombustion chamber 22.

In use, impingement holes 62 a and 62 b in the combustor dome allows airto pass into the cooling air space 60 between heat shield 40 back faceand the combustor dome panel. The air from combustor impingement holes62 of sets 62 a impinges upon the partial height pin fins 64 a inregions 80 a on the back face area of the heat shield 40 adjacentcircular ridge 66 a, and air from combustor impingement holes 62 of set62 b impinges upon the partial height pin fins 64 a in regions 80 b onthe back face area of the heat shield 40 adjacent ridges 66 b. Thepartial height pins 64 a provide sufficient clearance with the domepanel such that an optimal impingement height of 2-5 times the diameterof holes 62 is provided. After impinging the partial height pins 64 a,impingement air moves generally radially relative the heat shield, tomove past full height pin fins 64, in the case of air provided by holes62 of set 62 b. The splashed air from impingement holes 62 b is causedto flow over the pin fins towards the inner and the outer lips 41 and 43by the ridges 66, 69 and 71. This provides effective convection cooling.The air cools the back face of the heat shields by impingement andconvection heat transfer. The cooling air is eventually discharged fromthe space 60 behind the heat shield at the inner and outer lips 41 and43, where the flow may be straightened by the straightener ribs 72before being expelled into the combustion chamber 22 to traveldownstream along the inner and outer liners of the combustor. Oncetravelling along the combustor liners, dilution holes, etc. (not shown)redirect the flow into a double toroidal flow, as indicated by arrows a1and a2 in FIG. 2. Meanwhile, the majority of the air received withincircular ridge 66 a impingement cools the back face of the heat shield40 before flowing through the holes 67, preferably to provide cooling tothe upstream face of the heat shield. The remaining portion of the airreceived within circular ridge 66 a flows through grooves 63 to cool theback face of the heat shield radially outwardly therefrom, before beingdischarged radially at the outer lip 43.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without department from the scope of the invention disclosed.For example, the invention can be provided in any suitable heat shieldconfiguration and in any suitable combustor configuration, and is notlimited to application in turbofan engines. Still other modificationswhich fall within the scope of the present invention will be apparent tothose skilled in the art, in light of a review of this disclosure, andsuch modifications are intended to fall within the appended claims.

What is claimed is:
 1. A combustor and dome heat shield assemblycomprising a plurality of heat shield sectors mounted adjacent andspaced apart a distance from an annular dome of the combustor, each heatshield sector having a heat shield body defined between opposed lateraledges and radially inner and outer edges, at least one fuel nozzleopening defined in the heat shield body, the heat shield body having aback face facing the combustor dome, the back face having a firstplurality of protuberances extending from the back face to a firstheight, the first height being substantially equal to said distance, theback face having a second plurality of protuberances extending from theback face to a second height, the second height being less than thefirst height, the combustor having a plurality of impingement coolingholes defined through the dome, the impingement cooling holessubstantially aligned with the second plurality of protuberances.
 2. Thecombustor and dome heat shield assembly defined in claim 1, wherein aratio of a diameter of the impingement cooling holes to a height betweenthe impingement cooling holes and tops of the second plurality ofprotuberances is greater than one and less than five.
 3. The combustorand dome heat shield assembly defined in claim 2, wherein the ratio isgreater than one and less than three.
 4. The combustor and dome heatshield assembly defined in claim 1, wherein the second height issubstantially one half of the first height.
 5. The combustor and domeheat shield assembly defined in claim 1, wherein the protuberances arefrustoconical.
 6. The combustor and dome heat shield assembly defined inclaim 1, wherein the plurality of impingement cooling holes are definedin at least two rows, with at least one row disposed on each of opposedsides of the fuel nozzle opening, the rows defined on an arc concentricwith a central axis of the combustor.
 7. The combustor and dome heatshield assembly defined in claim 6, wherein the at least two rowscomprises four rows, two on each side of the fuel nozzle opening.
 8. Thecombustor and dome heat shield assembly defined in claim 1, wherein theheat shield body further comprises at least one ridge extending from theback face, and wherein the ridge extends along the back face adjacent atleast a portion of said second plurality of protuberances.
 9. Thecombustor and dome heat shield assembly defined in claim 8, wherein theplurality of impingement holes are defined in at least twonon-intersecting rows, and wherein the at least one ridge extends alongthe back face between the at least two non-intersecting rows ofimpingement holes.
 10. The combustor and dome heat shield assemblydefined in claim 1, wherein the second plurality of protuberances arelocated adjacent to ridges extending from the back face of the heatshield body.
 11. The combustor and dome heat shield assembly defined inclaim 1, wherein said first plurality of protuberances are provided inat least one central region and in radially peripheral regions, andwherein concentration density of said first plurality of protuberancesin said at least one central region is higher than in said radiallyperipheral regions.
 12. The combustor and dome heat shield assemblydefined in claim 11, wherein two central regions are provided, separatedby at least one laterally extending ridge, and wherein said radiallyperipheral regions comprise an inner peripheral region radially inwardlyof the at least one laterally extending ridge and an outer peripheralregion radially outwardly of the at least one laterally extending ridge.13. The combustor and dome heat shield assembly defined in claim 1,wherein the back face is divided in regions by ridges projecting alongsaid back face, said regions including radially inner corner regions andradially outer corner regions, and wherein said radially inner andradially outer corner regions are each provided with at least one row ofthe second plurality of protuberances adjacent the ridges separating theradially outer corners from the radially inner corners.
 14. Thecombustor and dome heat shield assembly defined in claim 1, whereinridges extend along the back face of the heat shield body, said ridgescooperating to direct a flow of cooling air to said radially outer andinner edges where the cooling air is discharged from the back face ofthe heat shield body, and wherein and array of straightener ribs isprovided along at least one of said radially inner and radially outeredges.
 15. The combustor dome and heat shield assembly defined in claim1, wherein a central circular ridge extends around the at least one fuelnozzle opening on the back face of the heat shield body, said centralcircular ridge circumscribing a central area, and wherein a plurality ofcooling holes are defined through the heat shield body in said centralarea.
 16. The combustor and dome heat shield assembly defined in claim15, further comprising diametrically opposed ridges running from saidcentral circular ridge towards said opposed lateral edges, a first pairof ridges running from the central circular ridge towards the radiallyouter edge in a direction generally perpendicular thereto, and a secondpair of ridges running from the central circular ridge towards theradially inner edge in a direction generally perpendicular thereto. 17.The combustor and dome heat shields assembly defined in claim 15,further comprising diametrically opposed ridges running from saidcentral circular ridge towards said opposed lateral edges, and whereinthe heat exchange promoting structures are arranged in rowssubstantially parallel to said diametrically opposed ridges.
 18. Thecombustor and dome heat shield assembly defined in claim 1, wherein theimpingement holes are arranged on circular arrays.
 19. A combustor domecomprising at least one heat shield mounted to an annular dome panel,the at least one heat shield and the dome panel defining an air spacetherebetween and at least one fuel nozzle opening for receiving a fuelnozzle, a plurality of impingement holes defined in said dome panel forcausing an impingement fluid to impinge upon a back face of the at leastone heat shield, the back face having a heat exchange promotingprotuberance pattern thereon, said pattern being divided in regions byribs projecting outwardly from said back face, said ribs including afirst rib extending around said fuel nozzle opening and circumscribing acentral annular area and a second rib extending from opposed sides ofthe first rib towards lateral edges of the at least one heat shield, theheat exchange promoting protuberance pattern including first and secondset of protuberances, the protuberances of the second set being shorterthan the protuberances of the first set and located at least partlyoutwardly of the central annular area adjacent to the first rib andadjacent to the second rib on either sides thereof and wherein theimpingement holes are substantially aligned with the second set ofprotuberances.
 20. The combustor dome defined in claim 19, wherein saidplurality of impingement holes comprises a first set of holes arrangedin at least one circular array around the fuel nozzle opening within thefirst rib for causing a cooling fluid to impinge upon said centralannular area, and a second set of holes circumferentially distributedabout a central axis of the annular dome panel, said second set of holesbeing disposed outside of said at least one circular array for directingthe cooling fluid upon the regions of the back face of the heat shieldlocated outside of said first rib.
 21. The combustor dome defined inclaim 19, wherein a ratio of a diameter of the impingement holes to aheight between the impingement holes and tops of the second set ofprotuberances is greater than one and less than five.
 22. The combustordome defined in claim 21, wherein the ratio is greater than one and lessthan three.
 23. The combustor dome defined in claim 19, wherein theheight of the protuberances of the second set is substantially one halfof the height of the protuberances of the first set.
 24. The combustordome defined in claim 19, wherein the plurality of impingement holes aredefined in at least two rows, with at least one row disposed on each ofopposed sides of the fuel nozzle opening, the rows defined on an arcconcentric with a central axis of the combustor dome.
 25. The combustorand dome heat shield assembly defined in claim 19, wherein said firstset of protuberances are provided in at least one central region and inradially peripheral regions, and wherein concentration density of saidfirst set of protuberances in said at least one central region is higherthan in said radially peripheral regions.